Active filter for resonance reduction

ABSTRACT

A control unit for an active filter for reducing resonance in an electric system is provided. The electric system comprises a power source distributing an alternating current to an AC conductor connected to a power consuming unit for distributing the AC to the power consuming unit. The active filter comprises a DC power source and a DC conductor connecting the DC power source to the AC conductor. The control unit comprises: a voltage measurement unit adapter to create a voltage signal on the basis of a measured voltage; a computing unit adapted to compute, using a biquadratic filter, a first compensating current on the basis of the voltage signal for reducing resonance in the electric system and a switching system placed between the DC power source and the DC conductor for creating the calculated first compensating current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 16/170,708,filed on Oct. 25, 2018, which is a divisional of application Ser. No.14/360,700, filed on May 27, 2014, now U.S. Pat. No. 10,141,741, issuedon Nov. 27, 2018, which is the National Stage entry under 35 U.S.C. §371 of International Application No. PCT/SE2012/051290, filed on Nov.22, 2012, which claims the benefit of Sweden Application No. 1151123-5,filed on Nov. 25, 2011, and U.S. Provisional Application No. 61/565,785filed on Dec. 1, 2011, the entireties of which are herein incorporatedby reference.

TECHNICAL FIELD

The invention relates generally to a method and arrangement for reducingresonance using active filters.

BACKGROUND

An electrical system or power grid comprises energy generating units andenergy consuming units or loads. The energy generating units (or in someinstances energy transforming units) in an Alternating Current (AC)system typically generates three alternating currents each having afrequency of 50 or 60 Hz and having an offset relative to each other of120°. To create a highly effective electrical system, the loads in thesystem should be purely resistive such that voltage and currentwaveforms are in phase and changing polarity at the same instance ineach cycle. With purely resistive impedance all energy generated in theenergy generating units will be turned into a useful and desired energyform at the load. However, in most cases, the impedance contains aninductive or capacitive component, which means that the current consumedis not always in phase with the supplied voltage of the alternatingcurrents. The inductive or capacitive components stores energytemporarily in electric or magnetic fields which is then returned to thepower grid a fraction of a second later in the cycle. This processresults in a time difference between the current and voltage waveformscreating nonproductive power which increases the current. Furthermore,many electrical apparatus comprises active components that have adynamically varying load, i.e. the load varies with time, such aselectrical apparatus converting electrical power to mechanical work.

In a normal AC electric system, the voltage varies sinusoidally at aspecific frequency, usually 50 or 60 hertz. known as utility frequency.A linear load, even if it comprises inductive or capacitive components,does not change the shape of the waveform of the current. However, theincreased use of pulse controlled apparatuses such as rectifiers,variable frequency drives or arc discharge devices, such as afluorescent lamps, electric welding machines, or arc furnaces introducesnon-linear currents into the power grid. Since currents in these systemsare interrupted by a switching action, the current contains frequencycomponents that are multiples of the utility frequency, known asharmonics. As nonlinear currents flow through an electrical system andthe distribution-transmission lines, additional voltage distortions areproduced due to the impedance of the loads and creates a currentwaveform which can become quite complex, depending on the type of loadand its interaction with other components of the system.

The harmonics are troublesome in many ways; one example is that electricmotors experience hysteresis loss caused by eddy currents set up in theiron core of the motor, which are proportional to the frequency of thecurrent. Since the harmonic frequencies are higher, they produce morecore loss in a motor than the utility frequency, which results inincreased heating of the motor core, which in turn shortens the life ofthe motor. Other problems with harmonics include overheating of cablesand transformers, damage to sensitive equipment and tripping of circuitbreakers.

Active filters capable of reducing the inductive and/or capacitivecomponents and harmonics are known in the art, for example in U.S. Pat.No. 7,245,045 to Ström et al., which is hereby incorporated byreference. An active filter is in principle a microprocessor controlledamplifier which is connected to the power grid, and which is arranged tosense and compensate the load's current consumption with regards tofrequencies which would not exist if the load was purely resistive.

Typically, an active filter comprises a main circuit with one or aseries of fast switches for each phase and each switch is connected to apower source, such as a Direct Current (DC) power source which canaccumulate electrical energy. The power grid's current provision and theloads current consumption are measured, and using Pulse Width Modulation(PWM) a compensating current is distributed to the power system by meansof the switches.

The current flowing in the electrical system is measured by means of acurrent measurement unit, after which it is transferred to a computingunit that typically transforms the measured current into a digitalfrequency domain signal using Fast Fourier Transformation (FFT). Thetransformed signal is used to create an inverted digital signal which isdistributed to the electrical system as a compensating current by meansof Pulse Width Modulation (PWM).

The active filters of the art comprise a filter circuit arranged inorder to reduce disturbances on the power grid generated by the activefilter. Since the active filter in principle is an amplifier, and sincethe filter circuit comprises an inductor and a capacitor it may form aresonant circuit together with the electrical system which amplifiesharmonic frequencies when resonance occurs. Resonance of the power gridmay be detrimental by causing unwanted sustained and transientoscillations, which in turn may cause performance degradation. The powergrid resonance at the resonant frequency may introduce substantialvoltage fluctuation. The effects of resonance in an electrical systemmay be progressively worsening as higher frequencies are generated.Especially critical for creating resonant behavior is harmonics of 7:thand 11:th order.

Active filters in the art measures the current flowing between the ACpower source and a power consuming device. Since the current flowing inthe conductors of the electrical system is a result of the voltage inthe power source and impedance of the consuming device, in accordancewith Ohm's law I=U/R, the measurement of the current will be delayed inrelation to fluctuations of the voltage level. On top of that, activefilters use an A/D conversion based on Fast Fourier Transform (FFT)which requires relatively computing intense calculations in themicroprocessor before a compensating current can be generated. Since theprocess of creating a compensating current is much too slow for handlingresonance, the response of active filters in the art to resonantbehavior in the electrical system has been to shut down the filter anddeal with the frequencies generating the resonance by adding componentsto the electrical system.

SUMMARY

It is an object of the embodiments herein to address at least some ofthe problems and shortcomings outlined above by using a method and anarrangement as defined in the attached independent claims.

A control unit for an active filter for reducing resonance in anelectric (or electrical) system is provided. The electrical systemcomprises a power source distributing an alternating current to an ACconductor connected to a power consuming unit for distributing the AC tothe power consuming unit. The active filter comprises a DC power source,and a DC conductor connecting the DC power source to the AC conductor.The control unit comprises a voltage measurement unit adapted to(measure a voltage, e.g. at the AC conductor, and to) create a voltagesignal on the basis of the measured voltage, a computing unit adapted tocompute a (first) compensating current on the basis of the voltagesignal for reducing resonance in the electric system, and a switch (orswitching) system placed between the DC power source and the DCconductor for creating the calculated compensating current. By measuringthe voltage a direct response is received which creates a system with alatency low enough to suppress resonance.

According to one embodiment of the control unit, the active filterfurther comprises a capacitor connected to the DC conductor and toground, being adapted to lead frequencies other than the utilityfrequency of the electrical system to ground. The voltage measurementunit is adapted to be connected to a point between the AC power sourceand the capacitor. By measuring the voltage at this location aninstantaneous value on the voltage level of potentially harmful currentsis collected since the measured voltage level instantaneously reflectsthe voltage level of the power system (or electric system).

According to an embodiment of the control unit, the computing unit mayuse a biquadratic filter (and, optionally, additional filters and/orprocessing means/steps) to compute the first compensating current on thebasis of the voltage signal. For example, the computing unit may applythe biquadratic filter (or a transfer function associated with thebiquadratic filter) to the voltage signal, or to a signal derived basedon the voltage signal. The biquadratic filter makes it possible to get asystem with a high sample frequency by means of standard components. Thebiquadratic filter could have a sample frequency exceeding 100 kSa/s(samples per second) or exceeding 150 kSa/s.

According to one embodiment of the control unit, the voltage measurementunit is further adapted to measure the voltage of a second and thirdalternating current having an offset in relation to the firstalternating current of substantially 120° (and 240° respectively, and tocreate a second and third voltage signal on the basis of the second andthird measured voltages. The computing unit may be further adapted tocompute a second and third compensating current on the basis of thesecond and third voltage signals. The DC power source may be adapted toreceive current from the second and/or third alternating currents, andthe switching system may be adapted to distribute the first compensatingcurrent to the first AC conductor (i.e. the AC conductor of the presentembodiment may e.g. comprise a first, second and third AC conductor towhich a first, second and third alternating current, respectively, isdistributed, and the first compensating current may be distributed tothe first of these AC conductors) by means of pulse width modulation.

According to an embodiment of the control unit, a current may bemeasured at the AC conductor, e.g. by a current measurement unit. Acurrent signal may be created based on the measured current and thecomputing unit may be adapted to compute the first compensating currenton the basis of the voltage signal and the current signal.

An active filter for reducing resonance in an electric system is furtherprovided. The electrical system comprises a power source distributing analternating current to an AC conductor connected to a power consumingunit for distributing the alternating current to the power consumingunit. The active filter comprises a DC power source and a DC conductorconnecting the DC power source to the AC conductor. The active filterfurther comprises the control unit according to any of the embodimentsherein.

A method of suppressing resonance in an electric system is furtherprovided. The method comprises: measuring a voltage of (or at: orrelated to) a conductor supplying energy from an AC source to an energyconsuming unit, creating a voltage signal on the basis of the measuredvoltage, computing a compensating current on the basis of the voltagesignal, and creating a control signal (on the basis of the computedcompensating current) for controlling a switching system to create thecompensating current by means of pulse width modulation.

According to one embodiment, the voltage is measured between theconductor and a capacitor adapted to lead alternating currents withfrequencies other that the utility frequency of the electric system toground.

According to an embodiment, the step of computing a compensating currenton the basis of the voltage signal may comprise using a biquadraticfilter (e.g. by applying the biquadratic filter to the voltage signal orto a signal derived from the voltage signal). The biquadratic filter mayhave a sample frequency of more than 100 kSa/s or more than 150 kSa/s.

A control method for an active filter connected to an electric system isfurther provided. The control method comprises measuring a voltage atthe electric system for deriving a current, comparing the derivedcurrent with a reference current, deriving a current error vector fromthe comparison and setting a current error threshold. The method furthercomprises creating a first current flow between a DC power source andthe electrical system based on the current error vector using a firstswitching pattern, when the current error vector is below the threshold,and creating a second, different, current flow between the DC powersource and the electrical system based on the current error vector usinga second switching pattern, when the derived current is above thethreshold. By providing a first and second switching pattern, a firstand second type of response may be created, one which is energyefficient, and one which is powerful enough to suppress resonance.

According to one embodiment, the electrical system is an alternatingcurrent three phase system comprising alternating currents having anoffset in relation to each other of substantially 120°, and wherein thethreshold is an upper and lower threshold that applies to all of thethree phases and thus creates a hexagonal threshold.

According to another embodiment, the first switching pattern is adaptedto create as long switching cycles as possible by creating a currentflow altering the direction of the current error vector within thehexagon towards the threshold furthest away.

According to yet another embodiment, the step of creating a secondcurrent flow between a DC power source and the electrical systemcomprises creating a current flow being the opposite to the currenterror vector such that the current error is reduced as efficiently aspossible.

According to one embodiment, the step of deriving a current error vectoris performed by using a biquadratic filter (e.g. by applying thebiquadratic filter). The biquadratic filter makes it possible to get asystem with a high sample frequency by means of standard components. Thebiquadratic filter could have a sample frequency exceeding 100 kSa/s orexceeding 150 kSa/s.

A control system for an active filter connected to an electric system isfurther provided. The control system comprises a measuring unit adaptedto measure the voltage at the electric system for deriving a current, acomparing unit adapted to compare the voltage of the electric systemwith a reference value for deriving a current error vector, a controlunit adapted to set a current error threshold, and a switch unit. Theswitch unit is adapted to create a first current flow between a DC powersource and the electrical system based on the current error vector usinga first switching pattern, when the current error vector is below thethreshold, and create a second, different, current flow between the DCpower source and the electrical system based on the current error vectorusing a second switching pattern, when the derived current is above thethreshold. By providing a first and second switching pattern, a firstand second type of response may be created, one which is energyefficient, and one which is powerful enough to suppress resonance.

According to one embodiment, the electrical system is an alternatingcurrent three phase system comprising alternating currents having anoffset in relation to each other of substantially 120°, wherein thethreshold is an upper and lower threshold that applies to all of thethree phases and thus creates a hexagonal threshold.

According to one embodiment, the first switching pattern is adapted tocreate as long switching cycles as possible by creating a current flowaltering the direction of the current error vector within the hexagontowards the threshold furthest away.

According to one embodiment of the control system, creating a secondcurrent flow between a DC power source and the electrical systemcomprises creating a current flow being the opposite to the currenterror vector such that the current error is reduced as efficiently aspossible.

According to one embodiment of the control system, the step of derivinga current error vector is performed using a biquadratic filter (e.g. byapplying the biquadratic filter).

Further possible features and benefits of this solution will becomeapparent from the detailed description below. Please note that anyembodiment or part of embodiment as well as any method or part of methodcould be combined in any way.

BRIEF DESCRIPTION OF DRAWINGS

Some possible embodiments will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a system overview of an active filter;

FIG. 2 is a system overview of an active filter for a three-phase systemshown in further detail;

FIG. 3a shows a graph of the three currents with a 120° phase shift;

FIG. 3b shows a graph of a utility frequency when modulated in relationto a perfect sinusoidal reference signal;

FIG. 4a shows a graph of a resulting error vector and a hexagon made ofthresholds;

FIG. 4b is a system overview of an active filter;

FIG. 4c is a graph of the error vector in relation to the hexagonthresholds:

FIG. 5a is a system overview of an active filter showing the units infurther detail:

FIG. 5b is a graph explaining the switching states;

FIG. 6 is a flow chart explaining a method of suppressing resonance inan electric system: and

FIG. 7 is a flow chart explaining a control method for an active filter.

DETAILED DESCRIPTION

An active filter is provided for reducing resonance in a power system byrapidly responding to resonant behavior and instantaneously providing acompensating current actively suppressing the resonance. The rapidresponse time is enabled by utilizing voltage measurement on a conductorbetween the electrical system to be monitored and compensated, and acapacitor adapted to attract alternating currents having frequenciesabove the utility frequency of the electrical system. By measuring thevoltage at this location an instantaneous value on the voltage level ofpotentially harmful currents is collected since the measured voltagelevel instantaneously reflects the voltage level of the power system, incontrast to currents which depend upon a current build up over theimpedance in response to a change in voltage.

The feeding of the compensating current, by means of switches and PulseWidth Modulation (PWM), is controlled based on time-domain signals, suchas the measured voltages, and the computing power intense step ofcreating a Fast Fourier Transform (FFT) of such time domain signals iseliminated. This reduces latency and allows for a more rapid response toresonant behavior by more rapidly providing a compensating currentactively suppressing the resonance

A modulation method based on two thresholds is further provided. The twothresholds enables the system to react in a first, less powerful, way aslong as the current error (deviation from the desired utility frequencycurrent) is kept below the first threshold, which optimizes theswitching for switching as few times per second is possible which makesthe active filter consume less energy and reduces the strain on theactive components, which prolongs the life of the active filter. As thecurrent error passes the first threshold, the filter responds verypowerfully by ignoring the amount of switches that needs to be made,instead the compensating current created is substantially the oppositeto the current error and thus reduces the current error as efficientlyand rapidly as possible, which enables the system to react to resonancesin the power grid excited by the load to be compensated.

In the following a detailed description of embodiments will be givenwith reference to the accompanying drawings. It will be appreciated thatthe drawings are for illustration only and are not in any wayrestricting the scope.

FIG. 1 shows an active filter 1 for resonance compensation according toa first embodiment in which the active filter 1 is in connection with anelectrical system 2. The electrical system 2 comprises an AC powersource 20 distributing an alternating current to an Alternate Current(AC) conductor 21 for transfer to a power consuming unit 22. The powersource 20 could be a generator producing the alternating current or atransformer used to transform the voltage level from a high voltagepower grid level to consumption level. The power consuming unit 22 isfor example an electrical machine transforming the electrical energysupplied from the power source 20 via the AC conductor 21 to mechanicalwork.

The active filter 1 is connected to the electrical system 2 via aconductor 3 placed between the active filter circuit 1 and the ACconductor 21, such that electrical current generated by the activefilter 1 can be distributed to the electrical system 2 for improving thecharacteristics of the alternating current flowing in the AC conductor21.

The active filter 1 comprises a Direct Current (DC) power source 10connected to a switch 11 (or switching unit/system) in a control unit16. The switch 11 creates a compensating current be means of Pulse WidthModulation (PWM) such that the average value of the current fed to theelectrical system 2 is controlled by the switch 11 being open and closedat a fast pace. The longer the switch 11 is closed compared to the openperiods, the larger the current supplied to the electrical system 2becomes.

Via a second conductor 15″, the switch 11 is connected to an inductor12. The inductor 12 transforms the pulses generated by the switch 11 toa continuous signal by opposing the changes in current through it bydeveloping a voltage across it proportional to the rate of change of thecurrent in accordance with the mathematical formula U=Ldl/dt. For anactive filter configured for 100 A current the inductor typically is aninductor in the range 200-250 uH. The inductor 12 is in turn connectedto the conductor 3 connecting the active filter 1 to the electricalsystem 2, such that the current created by the active filter 1 can besupplied to the electrical system 2. An additional conductor 15′″connects the conductor 3 to a capacitor 13 which in turn is connected toground 14. The capacitor 13 is set at a value allowing alternatingcurrents with a frequency at magnitudes above the utility frequency ofthe electrical current (for power grid applications typically 50-60 Hz)to pass to the ground 14 connection. This will effectively draw allfrequencies that create loss in the electrical system 2 to thecapacitor.

The control system 16 comprises a measurement unit 17′ connected via ameasurement conductor 18 to a point at the conductor 21; 3; 15′″ betweenthe AC power source 20 and the capacitor 13 such that the voltage levelof the components of the alternating current with frequencies above theutility frequency, passing through the conductor 21; 3; 15′″, will bemeasured with high accuracy and low latency.

The measured signal is transferred from the measurement unit 17′ to acomputing unit 17″ adapted to compute a compensating current forreducing frequency components of the current other than the utilityfrequency, further details with relation to the computation/creation ofthe compensating current will be described with reference to FIGS. 3-4.The computing unit 17″ is connected to a switching unit (or system)17′″, and the compensating current is realized by means of PWM by theswitch 11 of the switching unit 17′″, i.e. the compensating current issupplied from the DC source 10 and formed using PWM by the switchingunit 17′″.

FIG. 2 shows a version of the electrical system 2 similar to theelectrical system described with reference to FIG. 1. The maindifference being that the system according to FIG. 2 is a three phasesystem, such as most power grids, comprising three alternating currentseach with a phase offset of 120° and each connected to a three phaseload 22, for example by means of a Δ or Y coupling. The electricalsystem comprises 3 conductors 21 a; 21 b; 21 c for transferringelectrical current from the AC source 20 to the load 22.

Just like in the circuit described under reference to FIG. 1, the activefilter 1 comprises a direct current power source 10, here comprising acapacitor 37 in parallel with a resistor 38. The DC power source 10 isconnected to a switching unit 17′″ via conductors 32′; 32″. According tothe embodiment shown under reference to FIG. 2 the switching unitcomprises transistor bridges 31 a; 31 b; 31 c of pair-wise switches (oneof which is enlarged and indicated by reference numeral 31 a″) one topfor supplying positive voltages and one bottom for supplying negativevoltages for each of the three phases. Each switch, such as 31 a″,comprises a transistor 36, a diode 35 and a snubber 34, connected inparallel with the collector 36″ and emitter 36′″ of the transistor. Thetransistor 36 is the active component that controls the PWM, the diode35 directs current from the electrical system 2 to the DC accumulatorwhen the voltage of the electrical system 2 exceeds the voltage of theDC accumulator. The snubber 34 protects the transistor from shortvoltage spikes that are created when the switch turns off the current byabsorbing and reducing the voltage to manageable levels i.e. 50-100V.For each phase, the pair of switches are in one end connected to eachother and to the conductors 15″a; 15″b; 15″c, and their other endsconnected to the DC power source 10 by means of shared conductors 32′;32″. Both the specific design of the DC power source 10 and the switchesare to be seen as examples of implementations. The skilled personrealized that variations or alternatives to the designs are equallyconceivable.

The gate 36′ of the transistors 36 are connected to control leads 33 a;33 b; 33 c running from the computing unit 17″ such that the switchesoperate on pulses from the computing unit 17″ affecting thesemiconducting properties of the transistor 36. In the embodimentdescribed under reference to FIG. 2, the transistors are Insulated GateBipolar Transistors (IGBT) which are highly efficient and offers fastswitching, however it is conceivable that the transistors are of adifferent type, such as Metal Oxide Semiconductor Field EffectTransistors (MOSFET).

The inductors 12 a; 12 b; 12 c and capacitances 13 a; 13 b; 13 c arefurthermore adjusted such that the filter short-circuits harmonics anddisturbances which are generated by the switching of the transistorbridges 31 a; 31 b; 31 c are reduced/attenuated.

The control unit 16 of the active filter 1 encompasses a measuring unit17′ connected via measurement conductors 18 a; 18 b; 18 c to a point atthe conductors 3 a; 3 b; 3 c between the AC power source 20 and thecapacitors 13 a; 13 b; 13 c such that the voltage level of thecomponents of the alternating current with frequencies above the utilityfrequency, passing through the conductor 13 a; 13 b; 13 c will bemeasured with high accuracy and low latency. The measurement unit 17′ isconnected to a computing unit 17′ in which the Pulse Width Modulation(PWM) creating the control pulses to the switches is computed. Thecomputing unit 17″ comprises a processing unit, which may be a singleCPU (Central processing unit), or could comprise two or more processingunits. For example, the processor may include a general purposemicroprocessor, an instruction set processor and/or related chips setsand/or special purpose microprocessors such as ASICs (ApplicationSpecific Integrated Circuit). The processor may also comprise boardmemory for caching purposes.

As in FIG. 1 the switch system 17′″ is connected to inductors 12 a; 12b; 12 c which transform the pulses generated by the switch system 17′″to a continuous signal by opposing the changes in current through it bydeveloping a voltage across it proportional to the rate of change of thecurrent. The inductors 12 a; 12 b; 12 c are in turn connected toconductors 3 a; 3 b; 3 c connecting the active filter 1 to theelectrical system 2, such that the current created by the active filter1 can be supplied to the electrical system 2.

The measured signal is transferred from the measurement unit 17′ to acomputing unit 17″ adapted to compute a compensating current forsuppressing resonance and reducing frequency components of the currentother than the utility frequency. Further details with relation to thecomputation/creation of the compensating current will be described withreference to FIG. 3-4. The computing unit 17″ is connected to aswitching system (or unit) 17′″ that realizes the compensating currentby means of PWM.

Further details of the operation of the computing unit and the creationof the control signal for controlling the switches will now be describedunder reference to FIGS. 3-4.

According to at least some embodiments, the compensating current (orcurrents) will be created by means of one or more digital biquadraticfilters. Biquadratic filters are second-order recursive linear filtershaving a transfer function which is the ratio of two quadraticfunctions:

${H(z)} = \frac{b_{0} + {b_{1}z^{- 1}} + {b_{2}z^{- 2}}}{a_{0} + {a_{1}z^{- 1}} + {a_{2}z^{- 2}}}$

According to the embodiments described under reference to FIGS. 1 and 2the biquadratic filter is digitally implemented in the computing unit(reference number 17″) and typically samples the voltage signal with afrequency above 100 kSa/s, preferably above 190 kSa/s, which means thatsampling is more than 2000, preferably more than 3800, times faster thanthe utility frequency of the power system (or electric system) which isfast enough to react to resonant behavior of most harmonics in afrequency range that could cause problems. Furthermore, the biquadraticfilter has a very low latency which makes the reaction to resonantbehavior vary rapid which enables efficient suppression of the resonantbehavior.

According to the embodiments described with reference to FIG. 2, the DCpower source does not generate any current of its own; it merely storesenergy which it takes from the AC phases short periods of time such thatthe DC current should flow continuously. This means that the switches inthe switching system (further described under reference to FIG. 2) needsto be set such that power is received from one AC phase and thendistributed to another AC phase in accordance with a switching schedulewhich will be describes later.

The modulation will now be described in further detail.

FIG. 3a shows the utility frequency of the three AC phases AC1, AC2, AC3in the time/voltage domain. The modulation is hysteresis based such thatthe difference, or error, between the desired (reference) output currentand the actual output current is calculated for the purpose of creatingan error vector to be controlled.

FIG. 3b shows the actual utility frequency of AC1 when modulated inrelation to the perfect sinusoidal reference signal AREF. The modulationoperates by sending a compensating current out to the grid and therebyswitching the direction of the error vector which keeps the error withinthe thresholds T1 and T2. In normal operations, the error vector shouldbe kept within the first threshold T1 and the switching should beadapted such that each switch cycle is as long as possible, which keepsdown the power consumption of the switches and prolongs the life of theswitches.

FIG. 4a shows the reference current I_(setp) obtained by the controlunit 16 by applying one or more biquadratic filters to the current inthe electric system 2. The current on which the biquadratic filter isapplied, may be obtained from the measured voltage. Alternatively, thebiquadratic filter may be applied to the measured voltage and thereference current I_(setp) may then be obtained from the resultingfiltered voltage. The reference current I_(setp) represents resonantcomponents or other undesirable components, in the current in theelectric system 2, which are to be removed or compensated for by theactive filter. The biquadratic filters may have been tuned duringconfiguration or setup of the active filter to ensure that resonantcomponents end up in the reference current I_(setp) while the utilityfrequency of the electrical system (e.g. 50 or 60 Hz) is attenuated bythe biquadratic filters. FIG. 4a also shows the actual current I_(out)delivered by the active filter (via the switching system 17′″) to theelectric system 2. The difference between the reference current I_(setp)and the delivered current I_(out) is shown as an error I_(err) which isto be kept within a hexagon 64 of thresholds.

FIG. 4b is a simplified system illustration showing the control unit 16generating the reference current I_(setp), which after comparison withthe delivered current I_(out) generates the resulting current errorvector I_(err). The resulting current error vector I_(err) is thencompared with the first and second thresholds in the comparators (51a-51 d). In further detail the error vector I_(err) is compared to thesecond (outer) threshold I_(outer) in 51 a, the first (inner) thresholdI_(inner) in 51 b, the first negative (inner) threshold −I_(inner) in 51c in the second negative (outer) threshold −I_(outer) in 51 d. Thecomparison results in a control signal 33 for controlling the switchessuch that the most suitable compensating current is created.

FIG. 4c shows the inner 64 and outer 65 hexagon having an inner andouter threshold for the three phases (AC1, AC2, AC3) respectively. Thethresholds can be controlled by a digital potentiometer and tuneddepending on the system in which the active filter is placed, such thatthe response from the filter most efficiently suppresses the errors andwhilst consuming as little energy as possible.

In some embodiments, the reference current I_(setp) in FIG. 4b may bebased on both a measured current and a measured voltage. Biquadraticfilters may be applied to these measured signals (independently) and thereference current I_(setp) may e.g. be formed as a sum of thesecontributions.

FIG. 5a shows a circuitry diagram of an embodiment of the modulationsystem with error vectors 61 a, 61 b, 61 c of the three phases The errorvectors with top or bottom values 52 g-l are further provided as inputto the outer sector detector 65 adapted to determine whether or not theresulting error vector remains within the boundaries of the outerhexagon 65, which places the resulting error vector I_(err) in a sectorS-SVI of the hexagon 65 and provides output in the form of the sectorSI-SVI to a dynamic-state control unit 67 adapted to control the switchstates (of the switches for example shown as 31 a-31 c in FIG. 2) inaccordance with table 1 below: such that the switching is optimized foreffectively pushing the resulting error vector I_(err) back into theboundaries of the inner hexagon 64. The switching states of table 1 arerealized in accordance with the switching state graph in FIG. 5b .

TABLE 1 SI SII SIII SIV SV SVI ⁴Z ⁵Z ⁶Z ¹Z ²Z ³Z

The required switching states are received at a control signalgenerating unit 68 determining if switches need to be made and createsthe control signals 33 a′, 33 a″, 33 b′, 33 b″, 33 c′, 33 c″, receivedas input in the transistor bridges (31 a-31 c in FIG. 2) and controllingthe transistors such that a compensating current is created by means ofpulse width modulation.

FIG. 6 is a flowchart of a control method for an active filter accordingto one embodiment. The control method comprises the steps of measuringthe voltage 71 at a point between a capacitor and the electrical system(as further disclosed under reference to FIGS. 1 and 2), creating avoltage signal at a control unit 72 representing the voltage ofdifferent frequencies in the alternating current, computing 73, at acomputing unit, (based on the voltage signal and using a biquadraticfilter) a compensating current for suppressing resonant behavior and thevoltage level at frequencies other than the utility frequency, andcreating a control signal 74 on the basis of the voltage signal forcontrolling switches for generating the compensating current using pulsewidth modulation (also further disclosed with reference to FIGS. 1-4).

FIG. 7 is a flowchart describing a method of modulation according to oneembodiment. The method comprises the steps of measuring a voltage (81)at the electronic system for deriving a current (82). The voltage couldbe measured between a conductor of the electrical system and a capacitoradapted to lead alternating currents with frequencies other that theutility frequency of the electric system to ground, which creates a verygood connection between troublesome frequencies and the measurement. Themeasured voltage is used to derive a current which is then compared (83)with a reference current, having a perfect sinusoidal wave form suchthat a difference between the reference current and the measured currentcan be derived (84), the difference being a current error vector. Acurrent error threshold (85) is set in the control logic of the activefilter for defining to what extent a current error can be accepted anddefining what the response to the current error should be. The methodthen comprises the step of creating a first current flow (86) between aDC power source and the electrical system based on the current errorvector using a first switching pattern, when the current error vector isbelow the threshold (which is further explained under reference to FIGS.4c, 5a and 5b ). The method further comprises the step of creating asecond (87), different, current flow between the DC power source and theelectrical system based on the current error vector using a secondswitching pattern, when the derived current is above the threshold.

Pease note that any embodiment or part of embodiment as well as anymethod or part of method could be combined in any way. All examplesherein should be seen as part of the general description and thereforepossible to combine in any way in general terms.

What is claimed is:
 1. A control unit for an active filter for anelectric system, the electric system comprising an AC power sourcedistributing an alternating current to a first AC conductor, a powerconsuming unit connected to the first AC conductor and configured toreceive the alternating current from the first AC conductor, the activefilter comprising a DC power source, a DC conductor that connects the DCpower source to the first AC conductor, and a switching system placedbetween the DC power source and the first AC conductor, the switchingsystem being configured to create a compensating current based on areceived control signal, the control unit comprising: a voltagemeasurement unit adapted to measure a voltage of the first AC conductorand create a voltage signal on the basis of the measured voltage; and acomputing unit adapted to (i) receive the voltage signal, (ii) samplethe received voltage signal with a frequency exceeding 100 kSa/s, and(iii) compute the control signal on the basis of the sampled receivedvoltage signal.
 2. The control unit according to claim 1, wherein theactive filter further comprises a capacitor connected to the DCconductor and being adapted to lead frequencies other than a utilityfrequency of the electric system to ground, and wherein the voltagemeasurement unit is adapted to be connected to a point between the ACpower source and the capacitor.
 3. The control unit according to claim1, wherein the voltage measurement unit is further adapted to measure asecond voltage of a second AC conductor and a third voltage of a thirdAC conductor having a voltage offset in relation to the voltage of thefirst AC conductor of substantially 120° and 240° respectively, andcreate a second voltage signal on the basis of the second measuredvoltage and a third voltage signal on the basis of the third measuredvoltage, and wherein the computing unit is further adapted to sample thesecond and third voltage signals with a frequency exceeding 100 kSa/sand compute the control signal on the basis of the sampled second andthird voltage signals.
 4. The control unit according to claim 3, whereinthe DC power source is adapted to receive current from the second ACconductor and the third AC conductor to be distributed to the first ACconductor.
 5. The control unit according to claim 1, further comprisinga current measurement unit configured to measure a current of the firstAC conductor, and wherein the computing unit is adapted to compute thecontrol signal on the basis of the sampled received voltage signal andthe measured current.
 6. The control unit according to claim 1, whereinthe computing unit is adapted to sample the received voltage signal witha frequency exceeding 150 kSa/s.
 7. An active filter for reducingresonance in an electric system, the electric system comprising an ACpower source distributing an alternating current to a first ACconductor, a power consuming unit connected to the first AC conductorand receiving the alternating current from the first AC conductor, theactive filter comprising: a DC power source; a DC conductor thatconnects the DC power source to the first AC conductor; a switchingsystem placed between the DC power source and the first AC conductor,the switching system being configured to create a compensating currentbased on a received control signal, a voltage measurement unit adaptedto measure a voltage of the first AC conductor and create a voltagesignal on the basis of the measured voltage; and a computing unitadapted to (i) receive the voltage signal, (ii) sample the receivedvoltage signal with a frequency exceeding 100 kSa/s, and (iii) computethe control signal on the basis of the sampled received voltage signal.8. The active filter according to claim 7, wherein the active filterfurther comprises a capacitor connected to the DC conductor and beingadapted to lead frequencies other than a utility frequency of theelectric system to ground, and wherein the voltage measurement unit isadapted to be connected to a point between the AC power source and thecapacitor.
 9. The active filter according to claim 7, wherein thevoltage measurement unit is further adapted to measure a second voltageof a second AC conductor and a third voltage of a third AC conductorhaving a voltage offset in relation to the voltage of the first ACconductor of substantially 120° and 240° respectively, and create asecond voltage signal on the basis of the second measured voltage and athird voltage signal on the basis of the third measured voltage, andwherein the computing unit is further adapted to sample the second andthird voltage signals with a frequency exceeding 100 kSa/s and computethe control signal on the basis of the sampled second and third voltagesignals.
 10. The active filter according to claim 9, wherein the DCpower source is adapted to receive current from the second AC conductorand the third AC conductor to be distributed to the first AC conductor.11. The active filter according to claim 7, further comprising a currentmeasurement unit configured to measure a current of the first ACconductor, and wherein the computing unit is adapted to compute thecontrol signal on the basis of the sampled received voltage signal andthe measured current.
 12. The active filter according to claim 7,wherein the computing unit is adapted to sample the received voltagesignal with a frequency exceeding 150 kSa/s.
 13. A method of suppressingresonance in an electric system using an active filter, the methodcomprising: measuring a voltage of an AC conductor supplying energy froman AC power source to a power consuming unit at a point between the ACconductor and a capacitor adapted to lead frequencies other than autility frequency of the electric system to ground; sampling themeasured voltage with a frequency exceeding 100 kSa/s; and computing acontrol signal for a switching system of the active filter on the basisof the sampled measured voltage, for creating a compensating currentconfigured to suppress resonance in the electric system using the activefilter.
 14. The method according to claim 13, further comprising:measuring a current of the AC conductor; and computing the controlsignal on the basis of the sampled measured voltage and the measuredcurrent.
 15. The method according to claim 13, wherein the step ofsampling the measured voltage comprises sampling the measured voltagewith a frequency exceeding 150 kSa/s.
 16. A method of suppressingresonance in an electric system using an active filter, the methodcomprising: measuring a first voltage of a first AC conductor, a secondvoltage of a second AC conductor and a third voltage of a third ACconductor, the second and third voltage having a voltage offset inrelation to the first voltage of the first AC conductor of substantially120° and 240° respectively; creating a first voltage signal on the basisof the first measured voltage, a second voltage signal on the basis ofthe second measured voltage and a third voltage signal on the basis ofthe third measured voltage; sampling the first, second and third voltagesignals with a frequency exceeding 100 kSa/s; and computing a controlsignal for a switching system of the active filter on the basis of thesampled first, second and third voltage signals, for creating acompensating current configured to suppress resonance in the electricsystem using the active filter.
 17. The method according to claim 16,wherein the step of sampling the second and third voltage signalscomprises sampling the second and third voltage signals with a frequencyexceeding 150 kSa/s.